1. Field of the Invention
The present invention relates to a method for arc welding a very low carbon steel.
2. Description of the Prior Art
There has been a recent trend to reduce the carbon content in steel, especially for line pipes, off-shore structures and pressure vessels in low temperature use. The main reason for carbon reduction is the importance of weldability and fracture resistance in a weld heat-affected zone of steel. The remarkable advance of thermo-mechanical treatment in steel rolling has made it possible to reduce the carbon content to a very low level without any significant decreases in steel strength.
It has been common knowledge to welding engineers that steel with a lower carbon content is less susceptible to solidification cracking, which occurs in weld metal in arc welding by shielded metal arc welding (referred to as "SMAW" hereinafter) processes and gas metal arc welding (referred to as "GMAW" hereinafter) processes, than that with higher carbon. From this point of view, it has been an established recognition to set the carbon content of weld metal not higher than that of base steel, in order to improve weld metal toughness as well as resistance of weld metal to cold cracking and solidification cracking. No solidification cracking has been considered to occur as long as the carbon content of weld metal is less than 0.15%. No one has doubted that this concept is applicable to weld metal with a very low carbon content of less than 0.10%.
We, the present inventors, have investigated the solidification cracking susceptibility of weld metal using steel containing carbon in an amount of less than 0.06%. In this investigation, we have obtained results contrary to the above common knowledge. The result is that solidification cracking susceptibility has been increased despite reducing the carbon content. The solidification cracking has occurred in SMAW processes with welding speed higher than 30 cm/min, in GMAW using solid wires with the speed higher than 50 cm/min and in GMAW using flux cored wires with the speed higher than 40 cm/min, respectively.